1. Field of the Invention
The present invention relates to a controller and a control method for a homogeneous charge compression ignition engine which causes an air-fuel mixture formed in a combustion chamber to self-ignite by a compression of a piston under at least a part of operation conditions.
2. Description of the Related Art
In a homogeneous charge compression ignition engine, an air-fuel mixture formed by the mixture of air and fuel in a combustion chamber reaches a self-ignition temperature by the compression of a piston to simultaneously start combustions at a plurality of positions in a space inside the combustion chamber. Specifically, a electric spark (electric discharge), which is used for ignition in a conventional gasoline internal combustion engine, is not required in the homogeneous charge compression ignition engine.
A rise in temperature of the air-fuel mixture by the compression of the piston is brought about by an adiabatic compression action. Therefore, in order to obtain a greater adiabatic compression action so as to allow the temperature of the air-fuel mixture to reach the self-ignition temperature, a compression ratio is generally set higher in the homogeneous charge compression ignition engine than in a conventional spark-ignition internal combustion engine.
The self-ignition temperature of gasoline is about 300° C. although slightly different depending on a pressure and a mixture concentration. It is difficult to increase the temperature of the mixture containing a large amount of air at room temperature to about 300° C. only by the adiabatic compression action. Accordingly, it is necessary to increase the temperature of the mixture to be higher than that in the case of the conventional gasoline internal combustion engine by using means for causing a part of a combustion gas at a high temperature to remain in the combustion chamber.
As the means for causing a part of the combustion gas at a high temperature to remain in the combustion chamber, there is known a controller for an engine, for changing an internal exhaust gas recirculation (EGR) amount by changing at least one of valve-opening timings and valve-opening periods of an intake valve and an exhaust valve (for example, see Japanese Patent No. 4086602).
Specifically, for example, by adjusting a timing of closing the exhaust valve to the advance side, the amount of a high-temperature combustion gas, which is not discharged from the combustion chamber, increases. Therefore, the temperature of the mixture in the combustion chamber in the next cycle can be increased to be high.
The increase in temperature of the air-fuel mixture itself is relatively easily realized in the above-mentioned manner. However, if the temperature of the mixture becomes higher than needed, a combustion speed increases to cause combustion with vibrations similar to knocking. On the other hand, if the temperature of the mixture becomes lower than needed, the ignition becomes unstable to cause an accidental fire. Therefore, the temperature of the air-fuel mixture is required to be always controlled to be a proper temperature.
Moreover, in the conventional gasoline internal combustion engine, the air-fuel mixture is forcibly ignited by the electric spark (electric discharge). Therefore, in the case of common commercially-available gasoline, the air-fuel mixture can be combusted with little affected by a variation in composition of elements. On the other hand, in the homogeneous charge compression ignition engine without the electric spark (electric discharge), even in the case of the common commercially-available gasoline, the self-ignition temperature or the like changes depending on a difference in composition of elements, which is generated between different seasons, regions where crude oil is extracted, oil factories and the like. As a result, the temperature of the air-fuel mixture, which is suitable for the combustion, differs.
In order to cope with the above-mentioned problem, Japanese Patent No. 4086602 cited above describes a technology for controlling the temperature of the air-fuel mixture to a temperature suitable for the combustion with the current composition of elements of gasoline, based on the result of detection of a combustion timing or the like, specifically, for example, a measured value of a pressure in the combustion chamber.
Moreover, at the time of combustion of the air-fuel mixture in each cylinder, a crank angle (combustion timing) at which a heat generation rate (generated heat amount for each unit crank angle) becomes maximum, and a crank angle at which an ion current becomes maximum have a strong correlation. Based on the above-mentioned fact, a technology for controlling the temperature of the air-fuel mixture based on the ion current is known (for example, see Japanese Patent Application Laid-open No. 2008-291717).
However, the conventional technologies have the following problem.
The conventional gasoline internal combustion engine with the spark ignition is operated at a stoichiometric mixture (excess air ratio λ=1) at which a hydrocarbon-based fuel changes into carbon dioxide and water by an oxidation reaction without excess of oxygen, whereas the homogeneous charge compression ignition engine is operated in a super-excess air state at a multiple of the stoichiometric mixture, for example, at the excess air ratio λ=about 3 to 6.
The above-mentioned operation is performed to obtain a highly efficient combustion state over a wider range of load condition by reducing an intake pumping loss by fully opening a throttle under a low-load condition, and by suppressing a rise in combustion speed, in particular, under a high-load condition.
On the other hand, the ion current is electrons and cation molecules having a positive potential which are generated in the combustion reaction process of the hydrocarbon-based fuel, and negative ions and positive ions generated by thermal dissociation, which are captured by applying a high electric field so as to be detected as a current. Therefore, the detected ion current changes depending on a space density of the electrons and ions present in a space to which the high electric field is applied.
FIG. 6 is an explanatory view showing a change characteristic of a maximum ion current measured by the inventor (s) of the present invention with respect to the excess air ratio λ. The maximum ion current means a maximum value of the ion current, which is measured during one combustion cycle, and is generally a minute current on the order of microamperes.
In FIG. 6, when the homogeneous charge compression ignition engine is operated in the super-excess air state, for example, at the excess air ratio λ=3 or larger, the maximum ion current becomes minute to the level of noise of an electric circuit. Therefore, a timing at which the ion current becomes maximum cannot be detected. Therefore, there is a problem in that the temperature of the air-fuel mixture cannot be controlled to a temperature suitable for the combustion based on the ion-current information.